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Enhanced MJO and transition to superrotation in warm climates
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
Number of Authors: 22016 (English)In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 8, no 1, p. 304-318Article in journal (Refereed) Published
Abstract [en]

Using the NCAR CAM3 model in aquaplanet configuration, we perform a suite of simulations spanning a broad range of warm climates. The simulations show a spontaneous transition to superrotation, i.e., westerly winds at upper levels above the equator. The momentum convergence leading to superrotation is driven by eastward-propagating equatorial waves with structure similar to the modern Madden-Julian Oscillation (MJO), whose amplitude increases strongly with temperature. We analyze the moist static energy (MSE) budget of the model's MJO to identify mechanisms leading to its enhanced amplitude. Two such mechanisms are identified: a rapid increase of mean low-level MSE with rising temperature, as found in previous work, and reduced damping of the MJO by synoptic-scale eddies. Both effects imply a reduced gross moist stability and enhanced MJO amplitude. The reduced eddy damping is caused by the transition to superrotation, which allows a greater penetration of extratropical eddies into the equatorial zone; the dominant effect of this greater penetration is to flatten the meridional gradient zonal-mean MSE, which effectively impedes the generation of anomalous MSE divergence by MJO-modulated eddies. This mechanism implies a positive feedback between superrotation and the MJO which may hasten the transition into a strongly superrotating state.

Place, publisher, year, edition, pages
2016. Vol. 8, no 1, p. 304-318
Keywords [en]
MJO, superrotation
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-130981DOI: 10.1002/2015MS000615ISI: 000374770200015OAI: oai:DiVA.org:su-130981DiVA, id: diva2:936214
Available from: 2016-06-13 Created: 2016-06-09 Last updated: 2018-08-07Bibliographically approved
In thesis
1. Atmospheric dynamics and the hydrologic cycle in warm climates
Open this publication in new window or tab >>Atmospheric dynamics and the hydrologic cycle in warm climates
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Past warm climates represent one extreme of Earth's known climate states. Here, we study warm climates in both idealized simulations and full-complexity general circulation model (GCM) simulations of the early Eocene epoch, approximately 50 million years ago.

In increasingly warmer idealized aquaplanet simulations, the amplitude of intra-seasonal tropical variability is enhanced. The anomalies propagate eastward in the tropics and resemble the observed Madden-Julian Oscillation (MJO). The strong MJO anomalies drive a momentum convergence on the equator that causes westerly winds in the troposphere, a state known as superrotation. The results in this thesis show that superrotation further enhances the MJO by affecting the penetration of midlatitude eddies into the deep tropics. An additional question is how a super-rotating atmosphere, a dramatically different general circulation regime compared to today, will affect the climate, potentially via changes in cloud distributions and ocean circulation. If the superrotation extends down to the surface near the equator, surface westerly winds will drive equatorial downwelling in the eastern equatorial Pacific Ocean, rather than upwelling as in the present climate. Here, we show that surface superrotation is unlikely in past warm climates, although this in part depends on the intensity of the vertical momentum transfer associated with cumulus convection and how this process is represented in a specific GCM. 

There is, currently, no consensus on what the specific mix of forcings was that caused the warm climates of the early Eocene. High greenhouse gases likely played a significant role, but simulations with reasonable greenhouse gas concentrations cannot reproduce the high temperatures estimated by proxy data. Here, we investigate both an early Eocene climate forced by high greenhouse gas concentrations and one forced by optically thinner clouds, with artificially increased cloud droplet radius that causes increased solar radiation at the surface. Both alternative warming scenarios produce nearly identical zonal mean temperatures, but the hydrologic cycle differs; the thinner clouds scenario has 11% larger global mean precipitation. Moreover, the results in this thesis indicate that a reasonable estimate of vegetation, based on the model simulation, is likely necessary to evaluate alternative warming scenarios with proxy data.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2018. p. 30
Keywords
superrotation, early Eocene, warm climates, Madden-Julian Oscillation, the hydrologic cycle, vegetation sensitivity, large-scale circulation
National Category
Meteorology and Atmospheric Sciences Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-158487 (URN)978-91-7797-352-2 (ISBN)978-91-7797-353-9 (ISBN)
Public defence
2018-09-20, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2018-08-28 Created: 2018-08-07 Last updated: 2018-09-07Bibliographically approved

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